Today there was another promotion waiting in my promotion central. Called: "Unexpected bonus".

It's nothing really innovative:
Earn up to two free nights at any of our 3,600+ hotels worldwide with Marriott Rewards® Unexpected Bonus.

See Details
To earn your free nights:
Register by July 31, 2013
Stay twice at any participating hotel between May 15 and September 2, 2013
Earn one free night after every second paid stay, up to two free nights
Enjoy your free nights at any category 1-5 hotel worldwide
This bonus season, expect big things—and earn even bigger.

EDIT: Just read in the Megabonus thread that 'unexpected bonus' is the new name of the Megabonus. So actually this promotion is no side promotion.